Gravity is an important environmental factor to which terrestrial animals have adapted. Evolution into taller species requires cardiovascular adaptations such as: 1) higher arterial blood pressure to overcome greater vertical distances above heart level, 2) thicker vascular walls in legs to withstand greater intraluminal pressure, and possibly, 3) higher peripheral resistance in dependent tissues to prevent adema formation. The giraffe Giraffa camelopardalis is a unique animal in terms of extreme hydrostatic plressure gradients and natural adaptation to hypertension in its cardiovascular system. A five meter high adult has a total passive hydrostatic pressure gradient of 370 mm Hg. Added to a mean arterial blood pressure of 90 mm Hg at head level gives a theoretical mean blood pressure of 460 mm Hg in the feet. Although never measured directly, venous and capillary pressures in the foot probably range between 180-300 mm Hg since heart level is approximately equidistant between the head and foot; and therefore, the giraffe is susceptible to severe dependent edema. Although Otto Gauer speculated that high intravascular pressure in giraffes is opposed by high interstitial fluid pressure (the latter maintained by a tight giraffe skin), no study to date has elucidated the adaptive mechanisms involved. Therefore, the objective of this research is to study tissue fluid balance and edema prevention in lower legs of the giraffe. Transcapillary fluid balance will be studied in neck and leg tissues during conditions of: 1) standing with head upright and down drinking, 2) supine during anesthesia and normal sleeping, and 3) walking and running. The last condition will investigate effects of skeletal muscle and/or lymphatic pumping on tissue fluid balance. Recent development of wick techniques for measuring interstitial hydrostatic and colloid osmotic pressures allows direct investigation of the Starling pressures which govern transcapillary fluid balance in the lower leg and neck of the giraffe. Skin and muscle blood flows and lymph flow will be measured by isotope-washout techniques. Radiotelemetry of blood and interstitial pressures combined with studies of blood and lymph flows will yield quantitative data regarding mechanisms of edema prevention and blood pressure/blood flow regulation in an animal subjected to "natural hypertension." Microcirculatory morphometry will be correlated with blood pressure. The long-term objective is to investigate mechanisms which regulate transcapillary pressures and blood flow in dependent tissues in a cardiovascular system subjected to uniquely high and variable hydrostatic pressures.